In recent years optical communication technology and optical information processing technology have assumed a central role in a wide range of fields. Digital optical communication using optical fibers has made it possible to realize large increases in information processing in which the technology has been extensively utilized in the form of optical disks and laser printers and the like. Related fields of application are expanding at a remarkable pace.
Advances in laser diodes, which form the light source in such application, have played a major role in the development of such technologies. The small size and high efficiency of laser diodes have brought them into widespread use as laser light sources in compact disk, video disk, and optical communication systems.
In a laser diode the lasting action is generated by the injection of electrons into a P-N junction active layer. With recent advances in semiconductor technology, especially MBE (molecular beam epitaxy) and MOCVD (metal-organic chemical vapor deposition), enabling the controlled growth of epitaxial layers as thin as 10 A or less, have made it possible to utilize epitaxial layers with good uniformity to produce laser oscillation throughout the whole width of the gain region (active region) even when the region has a relatively large width, without giving rise to the phenomenon of filament oscillation.
FIG. 4(A) illustrates a prior art wide gain-region laser diode with an emission region 30 micrometers or more in width, using a refractive index waveguide type arrangement in which an active layer 10 is sandwiched between layers having different refractive index. Layer 10 has resonator end faces 12 and side faces 14. In a low power laser diode with an output in the order of 100 mW, the width W of the active layer 10 is made 10 micrometers or less so that it acts as a single mode optical waveguide, with a resonator length L of around 250 micrometers. In a wide gain-region laser diode, the width W is set at 30 micrometers or more to achieve a high output. Increasing the width W thus makes it possible to obtain an output of one watt or more compared to the output of 100 mW obtained with the usual width of 10 micrometers or less.
FIG. 4(B) shows the path of light in the active layer 10. Owing to the small L/W ratio, setting the width W to 30 micrometers or more to raise the output increases the probability that in addition to Fabry-Perot modes (indicated by the solid double headed arrows), modes generated between the resonator end faces 12 will also include modes produced by reflections from the side faces 14 of the active layer 10, which are indicated in the drawing by broken lines. As such modes including side wall reflections have oscillation patterns which differ from those arising from Fabry-Perot modes, they destabilize the oscillation pattern of the laser diode, making it very difficult to obtain uniform oscillation.